@TP-Link Thanks for the suggestion, I'll see how it goes for a few weeks. The CPU seems to be back to normal fluctuating around 30% since the reboot.

I fixed it with a reboot of all 3 devices, possibly a bug, since it was also holding the CPU at 100% constantly.

My Deco's are running in Access point mode. A while ago I enabled the guest WiFi to try it out, quickly found it doesn't segregate the traffic in Access point mode so didn't use it. But I forgot to...